In recent years, an electric vehicle has begun to spread instead of an automobile provided with an internal-combustion engine which causes a high environmental load due to exhaust gas. In anticipation of further spread of an electric vehicle, some inventors have tried to achieve effective utilization of electric power stored in an electric vehicle in each household. Here, DC power supplied from an electric vehicle is converted by a power conversion device into AC power linked with a power system, and then is supplied to a power load in household. Thus, so-called V to H (Vehicle to Home) is realized.
Similarly, a power control device (which is also called as a power conditioner) is also used for supplying electric power, which is generated by a power generation device such as a solar panel or cogeneration, to a power load connected with a power system. In this manner, a power control device plays an important role for linkage with a power system. However continuation of isolated operation in a state connected with a power system is inhibited when power failure occurs in the power system, in order to ensure safety. On the other hand, when assuming power failure due to a disaster or the like, it is preferable to break connection with a power system by a breaker and then supply electric power to a power load, so as to back up the supply of electric power.
For example, Japanese Patent Application Laid-Open No. 08-196040 discloses a dispersion type power source system which can cause a power conversion device (power control device) to perform self-sustaining operation when abnormality occurs in a power system and can return the operation of the power conversion device to linkage operation linked with the power system when the power system is returned to the normal state. Moreover, Japanese Patent Application Laid-Open No. 2006-254537 discloses technique wherein a power generation device linked with a power system can start and stop self-sustaining operation during power failure of the power system.